CNS Transplants and the Host Immune Response: The Blood-Brain Barrier and Immunological Privilege within the Mammalian Brain

Abstract

Survival of fetal mammalian brain tissue or of cell suspensions grafted into the adult mammalian brain is dependent upon the graft developing a vascular network supplied with host blood and failure of the host immune system to reject the graft as foreign tissue. The mammalian CNS has long been suspected to be a site of “immunological privilege” where constituents of the peripheral immune system do not venture. This suspected privilege is attributed to the absence of easily identifiable antigen presenting cells (e.g., cells expressing MHC class II antigen constitutively), the absence of lymphatic drainage, and the presence of the blood-brain barrier (for reviews see Head and Billingham, 1985; Widner and Brundin, 1988; Sloan et al., 1991).